For a long time I've been a proponent of a very simple method of adding coffee to your beer. My process is a long cold steep - 1 cup coarse ground coffee, 2-3 cups of filtered cold water. Mix in a French press, soak for 8 to 12 hours, press, filter and add to the beer in the keg/secondary.
I do this process a few times a week because it's a way of making coffee that my un-caffeinated brain can handle. Press, pour over ice with a little water dilute, drink, repeat until brain screams for mercy. One pot of coffee concentrate can last for several days in the fridge.